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Health effects of tobacco smoking
The health effects of tobacco are the circumstances, mechanisms, and factors of tobacco consumption on human health. Epidemiological research has been focused primarily on cigarette tobacco smoking, which has been studied more extensively than any other form of consumption.

Tobacco is the single greatest cause of preventable death globally. Tobacco use leads most commonly to diseases affecting the heart, liver and lungs, with smoking being a major risk factor for heart attacks, strokes, chronic obstructive pulmonary disease (COPD) (including emphysema and chronic bronchitis), and cancer (particularly lung cancer, cancers of the larynx and mouth, and pancreatic cancer). It also causes peripheral vascular disease and hypertension. The effects depend on the number of years that a person smokes and on how much the person smokes. Starting smoking earlier in life and smoking cigarettes higher in tar increases the risk of these diseases. Also, environmental tobacco smoke, or secondhand smoke, has been shown to cause adverse health effects in people of all ages. Cigarettes sold in underdeveloped countries tend to have higher tar content, and are less likely to be filtered, potentially increasing vulnerability to tobacco-related disease in these regions.

The World Health Organization (WHO) estimates that tobacco caused 5.4 million deaths in 2004 and 100 million deaths over the course of the 20th century. Similarly, the United States Centers for Disease Control and Prevention describes tobacco use as "the single most important preventable risk to human health in developed countries and an important cause of premature death worldwide." Several countries have taken measures to control the consumption of tobacco with usage and sales restrictions as well as warning messages printed on packaging.

Smoke contains several carcinogenic pyrolytic products that bind to DNA and cause many genetic mutations. There are 45 known or suspected chemical carcinogens in cigarette smoke. Tobacco also contains nicotine, which is a highly addictive psychoactive drug. When tobacco is smoked, nicotine causes physical and psychological dependency. Tobacco use is a significant factor in miscarriages among pregnant smokers, it contributes to a number of other threats to the health of the fetus such as premature births and low birth weight and increases by 1.4 to 3 times the chance for Sudden Infant Death Syndrome (SIDS). The result of scientific studies done in neonatal rats seems to indicate that exposure to cigarette smoke in the womb may reduce the fetal brain's ability to recognize hypoxic conditions, thus increasing the chance of accidental asphyxiation. Incidence of impotence is approximately 85 percent higher in male smokers compared to non-smokers, and is a key factor causing erectile dysfunction (ED).

A person's increased risk of contracting disease is directly proportional to the length of time that a person continues to smoke as well as the amount smoked. However, if someone stops smoking, then these chances gradually decrease as the damage to their body is repaired. The health risks of smoking are not uniform across all smokers. Risks vary according to amount of tobacco smoked, with those who smoke more at greater risk. Light cigarette smoking still poses a significant (though reduced) health risk, as does pipe and cigar smoking. Smoking so-called "light" cigarettes does not reduce the risk.

Tobacco use most commonly leads to diseases affecting the heart and lungs and will most commonly affect areas such as hands or feet with first signs of smoking related health issues showing up as numbness, with smoking being a major risk factor for heart attacks, Chronic Obstructive Pulmonary Disease (COPD), emphysema, and cancer, particularly lung cancer, cancers of the larynx and mouth, and pancreatic cancer. Overall life expectancy is also reduced in regular smokers, with estimates ranging from 10 to 17.9 years fewer than nonsmokers. About one half of male smokers will die of illness due to smoking. The association of smoking with lung cancer is strongest, both in the public perception and etiologically. Among male smokers, the lifetime risk of developing lung cancer is 17.2%; among female smokers, the risk is 11.6%. This risk is significantly lower in nonsmokers: 1.3% in men and 1.4% in women. If one looks at men who continue to smoke tobacco, the risk increases to one in six. Historically, lung cancer was considered to be a rare disease prior to World War I and was perceived as something most physicians would never see during their career. With the postwar rise in popularity of cigarette smoking came a virtual epidemic of lung cancer.